We have previously demonstrated the destructive effects of hyperoxia in the atmosphere of fetal mouse kidney and lung in organ culture. We have now demonstrated that the activity of superoxide dismutase in fetal tissues is less than in adult tissues. Vitamin E, presumably acting as an antioxidant seems to protect the fetal lung against the toxic effects of oxygen. Ca2 ion has been shown to be an essential determinant to normal fetal kidney and lung development. When Ca2 ion concentrations were lowered, development was hindered whereas calcium ionophore A23187 restores development at the same level of Ca2 ion. We shall study the morphologic and biochemical basis for these observations.